FIG. 1 illustrates an existing transmissive liquid crystal display (LCD) device 10. The transmissive LCD device 10 comprises a front polariser 12, a transparent mechanical substrate 14, a transparent unitary electrode 16, a nematic liquid crystal layer 18, a transparent pixellated electrode 20 comprising a plurality of distinct electrodes each of which has an associated pixel switch 24, a transparent mechanical substrate 22, a rear polariser 26 and a back light 28.
The front polariser 12 and the rear polariser 26 are arranged as cross-polarisers. The mechanical substrate 14 supports the transparent unitary electrode 16 and provides mechanical rigidity to the device. It is typically made of glass and supports the front polariser 12 on its upper surface and the transparent unitary electrode 16 on its lower surface. The mechanical substrate 22 has the plurality of pixel switches 24 defined on its upper surface. Each of the pixel switches is electrically connected to one of the plurality of electrodes of the transparent pixellated electrode 20. The lower surface of the mechanical substrate 22 supports the rear polariser 26. The nematic liquid crystal layer 18 is positioned between the transparent unitary electrode 16 and the transparent pixellated electrode 20. When a pixel switch is on, a voltage is applied across the overlying portion of the liquid crystal layer 18. The back light is positioned beneath the rear polariser 26. Light from the back light 28 is initially polarised to a particular orientation by the rear polariser 26. As this light travels through the nematic liquid crystal layer 18 its polarisation will be changed again to either a first orientation or a second orientation depending upon whether the portion of the liquid crystal through which it is travelling has a voltage across it. The front polariser 12 will only let light pass through having polarisation of the first orientation. Therefore, the transmissive liquid crystal device 10 is selectively transmissive in dependence upon the state of the pixel switches 24. This type of device has good image quality in low and at normal ambient light conditions but it is very difficult to see in bright conditions. In addition, the device requires a permanent back light which uses too much power for many modern applications.
FIG. 2 illustrates an existing reflective liquid crystal display device 30. The reflective LCD device 30 comprises a front light 38, a front polariser 12, a mechanical substrate 14, a transparent unitary electrode 16, a nematic liquid crystal layer 18, a pixellated reflective electrode 32 comprising a plurality of distinct reflective electrodes each of which has an associated pixel switch 34 and a mechanical substrate 36.
The differences between the reflective LCD device 30 and the transmissive LCD device 10 are that the reflective device 30 has a front light 38 positioned above the front polariser 12 and does not have a back light; the pixellated electrode 32 between the lower mechanical substrate 36 and the liquid crystal layer 18 is reflective and is not transparent; and the absence of a rear polariser in the reflective LCD device 30.
Light from the front light 38 passes through the front polariser 12 so that it is polarised in a first orientation, when the light is reflected by a pixellated electrode its polarisation is changed. The polarisation of the light is further changed by passing through the liquid crystal layer 18 and the extent of this change is dependent upon the voltage applied across the portion of the liquid crystal layer through which the light travels, which is in turn dependent upon whether the pixel switch associated with that portion of the liquid crystal layer 18 is on or off. Depending upon the extent to which the liquid crystal layer changes the polarisation of the reflected light, the front polariser 12 either will or will not pass the light. Thus the pixel switches 34 can be used to make portions of the display either reflective or absorptive. A problem with this type of device is that the image quality of the display is reduced by the presence of a front light 38.
Transflective liquid crystal display devices address this problem by removing the front light 38 and introducing a back light. Each of the distinct electrodes making up the pixellated reflective electrode layer 32 is adapted to have a small hole in it which lets light through from the back light. However, transflective liquid crystal display devices are a poor compromise between transmissive and reflective liquid crystal displays. If the display is optimised towards being bright in daylight by making the holes in the reflector smaller, then it will be darker with the back light on or will need more power in the back light to get the same brightness.
It would be desirable to provide a display device which has good image quality irrespective of the ambient light conditions and does not have excessive power requirements.